The present invention relates to a mass spectrometer having an ionization chamber in which a sample is ionized under a pressure as high as near atmospheric pressure. Mass spectrometers of this type include, for example, an Inductively Coupled Plasma Mass Spectrometer (ICP-MS), an ElectroSpray Ionization Mass Spectrometer (ESI-MS), an Atmospheric Pressure Chemical Ionization Mass Spectrometer (APCI-MS).
FIG. 7 schematically shows the construction of a conventional electrospray ionization mass spectrometer. The mass spectrometer includes an ionization chamber 10 provided with a nozzle 11 connected to, for example, the outlet of a column of a liquid chromatograph, and an analyzing chamber 18 in which a quadrupole filter 19 and an ion detector 20 are accommodated. A wall separates the space between the two chambers 10, 18 into two parts, which are referred to as the first and second interface chambers 12, 15. The ionization chamber 10 and the first interface chamber 12 communicate only through a heated capillary 13, which is a pipe of a small inner diameter. The first interface chamber 12 and the second interface chamber 15 communicate only via skimmer 16 having an orifice 16 of a very small diameter.
The pressure in the ionization chamber 10 is maintained at about the atmospheric pressure by a continuous supply of a sample gas from the nozzle 11. The first interface chamber 12 is evacuated with a rotary pump (RP) so that the inside is kept at a low vacuum of about 102 Pa. The second interface chamber 15 is evacuated with a turbo molecular pump (TMP) so that the inside is kept at a middle-vacuum of about 10xe2x88x921 to 10xe2x88x922 Pa, and the analyzing chamber 18 is evacuated with another turbo molecular pump (or the same TMP mentioned above) so that the inside is kept at a high-vacuum of about 10xe2x88x923 to 10xe2x88x924 Pa. Thus, the analyzing chamber 18 is maintained at the high vacuum by decreasing the pressure gradually from the ionization chamber 10 to the analyzing chamber 18.
In an electrospray method, sample liquid is sprayed from the nozzle 11 into the ionization chamber 10 and the sample molecules are ionized when the solvent contained in the fine liquid particles vaporizes. The mixture of the liquid particles and the ions are drawn into the capillary 13 due to the pressure difference between the ionization chamber 10 and the first interface chamber 12, where the ionization further proceeds when the mixture flows through the capillary 13. The first interface chamber 12 is provided with a ring electrode 14 inside, which generates an electric field for assisting the drawing-in of ions to the capillary 13 for converging ions to the orifice of the skimmer 16.
The ions introduced through the orifice of the skimmer 16 into the second interface chamber 15 are converged and accelerated by an ion lens 17, and enters the analyzing chamber 18. In the analyzing chamber 18, only ions of a particular mass number (i.e. ratio of mass (m) 10 to charge (z), m/z) pass through the longitudinal space around the central axis of the quadrupole filter 19. Ions passing through the quadrupole filter 19 are detected by the ion detector 20.
The ion lens 17 in the second interface chamber 15 generates an electric field to accelerate and converge travelling ions as described above, and various types of ion lenses have been proposed conventionally. FIG. 8 is a perspective view of one of such lenses, a so-called electrostatic lens. The ion lens 21 shown in FIG. 8 is composed of plural lens electrodes made of ring metal plates. The lens electrodes are applied the same DC voltage. When the DC voltage is determined appropriately, ions travelling through the ion lens 21 on or near the ion beam axis C are accelerated. The ion lens, however, is deficient in that the converging efficiency is not very high, especially when the pressure is as high as 10xe2x88x921 Pa or higher. Accordingly, when, for example, ions travelling through the ion lens disperse, only a part of the ions pass through the ion lens and enter the section behind.
FIG. 9 shows another type of practically used ion lens, a so-called multi-pole type. The ion lens 22 shown in FIG. 9 is composed of four rod electrodes, but the number of rod electrodes may be any number so long as it is even. The rod electrodes are applied the same DC voltage and a high frequency AC voltage superimposed on it, where the phases of the high frequency AC voltages of adjacent rod electrodes are reversed. Electric field generated by the rod electrodes influences the ions introduced along the ion beam axis C so that they oscillate while travelling through the ion lens 22. By this type of ion lens, the converging effect of ions is very high, so that more ions pass through the ion lens and enter the section behind.
This type of ion lens, however, is also deficient in that ions are not accelerated while travelling in the space surrounded by the rod electrodes, since the potential gradient in the longitudinal direction of the space is zero. Therefore, when the ion lens is used under a condition where the pressure is as high as in the first interface chamber 12, only a small number of ions can pass through the ion lens, because the ions lose their kinetic energy as they collide with molecules of gas in the chamber.
With regard to the above-described problem, one object of the present invention is to propose a mass spectrometer having an ion lens whereby the convergence and acceleration of ions are performed effectively even under a pressure as high as near atmospheric pressure.
Thus, the present invention proposes a mass spectrometer having an ion lens for converging ions, characterized in that the ion lens is composed of an even number of virtual rod electrodes positioned separately around the ion beam axis, where each of the virtual rod electrodes is composed of a plurality of separate metallic plate electrodes aligned in a row, and a voltage is applied to each of the plate electrodes.
In the above-described mass spectrometer, the voltage applied to each of the plate electrodes constituting a virtual rod electrode is determined with respect to the position of the plate electrode in the virtual rod electrode. For example, when a voltage composed of a DC voltage and a high frequency AC voltage superimposed thereon is applied to each of the plate electrodes, the DC voltage may be changed according to the position of the plate electrode while the high frequency AC voltage is set at the same irrespective of the position. The high frequency AC voltage applied to a virtual rod electrode should be reversed in phase against that applied to the adjacent virtual rod electrode.
When ions produced in an ionization chamber enter the ion lens, the ions travelling through the ion lens oscillate transversally due to the electric field generated by the high frequency AC voltage, and converge on a focal point of the ion lens. Meanwhile, the voltage gradient due to the change in the DC voltage applied to the plate electrodes accelerates the ions. Thus, the ions keep travelling without being displaced too much from due converging paths even when they collide with molecules of residing gas. Therefore, when, for example, a skimmer having is set behind the ion lens so that the orifice is positioned at the focal point of the ion lens, a large number of ions can pass through the orifice and enter the section behind it.
Thus, by the mass spectrometer according to the present invention, the convergence and acceleration of ions are effectively performed even when the pressure is as high as near atmospheric pressure. As a result, an adequate amount of ions can enter the mass filter set behind the ion lens, and the sensitiveness and accuracy of the mass spectrometry are improved. Also, according to the present invention, various forms of electric field that are hardly realized by conventional solid electrodes can be realized without difficulty.
When, in the above-described ion lens, an ion has a relatively large kinetic energy, the ion is hard to converge and, accordingly, the probability of the ion""s passing through the ion lens is relatively low. Such a characteristic of the ion lens should be considered especially when atmospheric pressure chemical ionization method is used. That is, by atmospheric pressure chemical ionization, speed of ions is accelerated by a jet of nebulizer gas ejected at a constant speed. In this case, the initial kinetic energy of an ion is greater as the mass of the ion is larger. Therefore, the probability of an ion""s passing through the ion lens differs depending on the mass, which may yield an error in the result of mass spectrometry.
With regard to the above-described problem, the mass spectrometer according to one aspect of the present invention is constituted so that the voltage applied to a part of the plate electrodes is changed according to the mass number of ions intended to pass through the ion lens. For example, when a combination of a DC voltage and a high frequency AC voltage is applied to each of the plate electrodes, the DC component of the voltage applied to the last one or ones of the plate electrodes nearest to the exit of the ion lens is changed according to the mass number of the ions intended to pass through the ion lens.
In the above-described mass spectrometer, the rate of acceleration of ions travelling through the plate electrodes nearest to the exit of the ion lens can be controlled by changing the DC component of the voltage applied to them. When the mass spectrometer uses a quadrupole filter placed behind the ion lens, the DC component of the voltage applied to the plate electrodes may be preferably scanned synchronous to the scanning of voltage applied to the quadrupole filter. By controlling voltage as described above, the speed of ions having a greater kinetic energy due to a large mass number is relatively reduced, so that the ions are converged to the hole or orifice of the skimmer and enter the section behind.
Since, by the mass spectrometer constituted as described above, the convergence of ions are performed appropriately with respect to the mass number of the ions, an adequate amount of ions enter the section behind, irrespective of the mass number of the ions. Thus the accuracy and reproducibility of analysis is improved.